Half-square triangle size.

Answer

For a 3 in finished half-square triangle and a 1/4 in seam, cut squares 3 7/8 in. The common rule is cut size = finished size + 7/8 in. The exact figure is 3.854 in, and rounding up to the next 1/8 in gives the 7/8 in rule.

Worked example

Finished size 3 in, seam allowance 0.25 in. Exact cut size = 3 + 0.25 x (2 + 1.4142) = 3.854 in, rounded up to 3 7/8 in. The unfinished unit measures 3.5 in (3 + 2 x 0.25) before it is sewn in. For 10 HSTs you cut 5 squares from each of two fabrics: each pair makes two units. Check the other way: squares cut at 3 7/8 in finish at 3.021 in, a hair over 3 in, which is why the units are trimmed.

How it works

Place two squares right sides together, draw a diagonal, sew a seam allowance on each side of it and cut on the line. That gives two HST units. Because the seam sits a seam allowance away from the cut corner, the square must be bigger than the finished unit plus its two seam allowances. Along the diagonal the extra is the seam allowance times (2 + the square root of 2), so cut size = finished size + seam allowance x 3.414. That is this site's own geometry; the common published rule for a 1/4 in seam is to add 7/8 in (USU Extension), which is the same figure rounded up to the next 1/8 in. Rounding up leaves a little extra to trim the finished unit to size.

Larger HSTs. For a 6 in finished unit the cut size is 6.854 in, so cut 6 7/8 in squares.

Sources (opened 2026-10-08): Half Square Triangles, Broken Dishes Block, Pinwheel Block (USU Extension).

FAQ

How big do I cut squares for 3 inch half-square triangles?

Cut 3 7/8 in squares for 3 in finished HSTs with a 1/4 in seam. The rule is finished size plus 7/8 in; the exact value is 3.854 in.

Why add 7/8 inch to the finished size?

The seam is sewn a quarter inch from the diagonal cut, and the unit also needs a quarter inch seam allowance on every side. Together the extra is 0.25 x (2 + the square root of 2) = 0.854 in, which rounds up to 7/8 in.

How many squares do I need for a number of HSTs?

Each pair of squares, one from each fabric, makes two HST units. Divide the units you need by 2 and round up to get the squares to cut from each fabric.

Does this work for a seam other than 1/4 inch?

Yes. The calculator uses cut size = finished size + seam allowance x 3.414, so enter your own seam allowance. The 7/8 in rule only applies to a 1/4 in seam.

Should I trim HST units after sewing?

Many quilters cut a little oversize and trim each unit to the unfinished size (finished size plus two seam allowances). Rounding the cut size up to the next 1/8 in leaves room for that.